This invention relates to a chain tensioner mainly used to maintain tension of a timing chain for driving camshafts of an engine.
In a typical engine, the rotation of its crankshaft is transmitted to camshafts through a timing chain received in an engine cover to rotate the camshafts, thereby opening and closing valves mounted to combustion chambers. In order to keep the tension of the chain within a proper range, a tension adjusting device is typically used which comprises a chain guide pivotable about a shaft, and a chain tensioner for pressing the chain through the chain guide.
Among chain tensioners mounted in such tension adjusting devices, one as disclosed in JP utility model publication 64-41756 is known which includes a cylindrical housing having a closed bottom and having an internal thread formed on its inner periphery and in engagement with an external thread formed on the outer periphery of a rod for pressing the chain. A spring is disposed between the bottom of the housing and the rod to bias the rod away from the bottom of the housing.
When the tension of the chain changes while the engine is running, the rod repeatedly moves back and forth, while slowly rotating, within the range of the gap between the internal and external threads. The rod thus axially moves to a position where the biasing force of the spring balances with the tension of the chain.
When the engine is stopped, even if the tension of the chain is greater than the biasing force of the spring, because the chain does not vibrate, the rod neither rotates nor axially moves. Thus, when the engine is restarted, the chain is less likely to slacken, which permits a smooth restart of the engine.
But with this chain tensioner, because the rod does not rotate unless the chain is vibrating even when the tension of the chain increases, it is impossible to sufficiently reduce the tension of the chain if the engine resonates or fluctuations in torque of the crankshaft or rotational resistance of the camshafts per combustion cycle are large. Thus, the tension of the chain may increase excessively.
An object of the present invention is to provide a chain tensioner which can effectively reduce the tension of the chain while the engine is running.